The long-term objectives of the present research are to systematically investigate the variability of blood pressure readings in normotensive and hypertensive populations and to contribute to the literature on the factors explaining the variability in blood pressure readings in these populations. Two major studies are proposed. The first major study is a naturalistic study utilizing an automatic, non-invasive blood pressure recorder to investigate the 24-hour variation in the blood pressure readings of normotensive, labile hypertensive, and sustained hypertensive individuals across time, setting, molar behavior, persons present, and level of perceived stress. The second is an experimental investigation of the effects of setting and type of person monitoring the blood pressure on the blood pressures of normotensive and hypertensive individuals. Within each diagnostic group, normotensive and hypertensive subjects will be randomly assigned to a condition in which their blood pressures will be monitored by either a medical staff person, a non-medical research assistant, or the subject himself/herself in both a medical and home setting. The present research also proposes to: (1) identify factors which might explain variability in blood pressure and will consider as possible factors time since awakening, setting, person recording the blood pressure, anxiety, anger, Type A behavior, molar behavior, persons present, exercise habits, and various demographic factors; (2) determine whether there are differences in the variability of blood pressure readings between nonmotensive and hypertensive populations; and (3) determine whether there are differences in the factors which might explain variability in the blood pressure readings of normotensive and hypertensive populations.